In telescopic astronomy it is frequently useful to observe objects using optical filters. Filters are particularly useful for observing objects at specific wavelength bandwidths. Additionally, filters are useful for reducing the overall level of light observed making them advantageous for observation of very bright objects such as the sun.
When attempting to observe objects at specific wavelengths, filters having highly precise bandwidths and passbands are required. Unfortunately, it has proven difficult to mass produce etalon filters having the requisite optical specifications. Furthermore, etalon filters typically suffer from thermally induced variations from the desired performance specifications.
One type of useful filter is the Fabry-Perot etalon filter. Although an excellent filter, it has not yet been possible to mass produce etalons having sufficient quality. Until now, the construction of such etalons has thus far been a highly sensitive "craft" able to manufacture only a few units at a time. As a result, etalons of this type are extremely expensive.
The characteristics of Fabry-Perot etalons are well known in the art and are discussed in a number of classic texts. For example, M. Born and E. Wolf, "Principles of Optics" Pergamon Press (1980) incorporated herein by reference. In general, a Fabry-Perot etalon consists of two parallel optically flat surfaces separated by a gap. The two surfaces may have an optical coating applied to their surfaces or may be uncoated. The surfaces can be the opposing faces of two separate plates separated by a gap, the gap being filled with air or a vacuum. Such an etalon is referred to as an "air-spaced etalon". An etalon may also be constructed using two parallel surfaces on opposite sides of a single solid plate. This is referred to as a "solid etalon". Both types are used extensively in spectral analysis, laser-line narrowing, mode selection, and as integral components in the construction of ultra-narrow band optical filters, as well as many other instances where spectral selection and filtering is desired.
An air-spaced etalon can be made extremely thermally stable, whereas a solid etalon is subject to changes in its optical thickness depending on changes in ambient temperature, thereby causing the passband of the etalon to change with changing temperature. Because it is desirable to have a stable passband, such solid etalons are not desirable.
Historically, air-spaced etalons have been constructed using two different designs. FIG. 1 shows an etalon 1 having two parallel optically flat surfaces (also called plates or etalon plates) 10, 11 separated by spacers 14, 15, which define a gap 17 equal to the thickness of the spacers 14, 15. As the ambient temperature changes, the spacers 14, 15 expand and contract leading to an expansion and contraction of the gap 17 which changes the passband of the etalon 1.
FIG. 2 illustrates an alternative design known as a "re-entrant" etalon 2. Such etalons feature a third plate known as a "riser" 19. Typically, re-entrant etalons 2 are used when a gap 17 of less than about 0.5 millimeters (mm) is desired. The gap 17 in such structures is defined by the difference in length between the spacers 14 and 15 and the thickness of the riser 19. The optical quality of the etalon 2 and consequently its efficiency is governed by two factors, the flatness of the plates 10, 11 and the parallelism of the gap 17. In any etalon, the flatness of the plates is a limiting parameter. In the air-spaced etalon, the parallelism of the gap is controlled by the ability to form spacers demonstrating adequate parallelism and, in the case of the re-entrant design (FIG. 2), the ability to form an adequately parallel riser 19.
The chief difficulty in manufacturing telescopic etalon filters is that to meet the necessary optical tolerances, extremely precise, time consuming, and expensive manufacturing techniques must be used. Although it may be possible to hand-manufacture small numbers of air-spaced etalons of the type described above, such techniques are so specialized that only a few persons in the world can make such filters. These techniques are more in the nature of an art and not at all suited to mass production. In fact, there is no presently known method for constructing such devices in large quantities. What is needed is an etalon having a high degree of optical precision and a high degree of thermal stability as well as a method of mass producing such etalons.